The advent of low voltage digital circuitry in present day electronics resulted in the use of many power converters which, for example, take the input power supply, ex. battery power, and down convert the battery voltage to a voltage suitable for operation of the electronic circuitry. Switch mode power supplies are needed and used in many electronic devices as they are more efficient in power conversion than linear regulator power supplies. Switch mode power supplies can be either magnetic based or capacitor based.
FIG. 1A shows a magnetics (inductor L1 106) based synchronous step down (Buck) DC-DC converter, which converts the input voltage VIN 103 to a lower output voltage VOUT 108. Transistors 104 and 105 are controlled by gates 101 and 102 respectively, to drive SW 119. Inductor L1 106 output VOUT 108 is filtered by capacitor 107 to supply LOAD 109.
FIG. 1B shows a magnetics (inductor L2 113) based step up (Boost) converter which converts VIN 111 to a higher voltage at the output VOUT 117. Transistors 112 and 114 are controlled by gates 110 and 115 respectively, to drive SW 120. VOUT 117 is filtered by capacitor 116 to supply LOAD 118.
Magnetics based converts are efficient but need magnetic components like Inductors and transformers which are bulky. In addition to making electronic devices bigger, the bulky magnetic components, due to their larger height, have a further disadvantage that they cannot be co-packaged with the components they are powering. As the converters are placed further away from the components they are powering, the increased parasitic trace resistances and capacitors further decrease the conversion efficiency and bandwidth. Furthermore, as the current in the magnetic component cannot be changed instantaneously, the presence of magnetic components limits the loop bandwidth of the converter.
Another conversion approach is the use of a switch capacitor based converter. FIG. 2A shows a ½ step down converter and FIG. 2B shows a 2× step up converter. This type of converter does not need the bulky magnetic components like inductors. However, switched capacitor based converters are only efficient when the output is a ratio of the input voltage and in addition, the output voltage regulation is poor thus requiring post regulation.
For example, in the switched capacitor converter of FIG. 2A, the output voltage VOUT 208 is equal to the half of the input voltage VIN 201. If VIN, as an example, is a Li-Ion battery, the voltage VIN 201 could be varying between 3V-4.3V. If the required output at VOUT 208 is 1.8V and when the voltage at voltage VIN is 4.3V, the switched capacitor converter ideally provides a voltage of 4.3V/2=2.15V at VOUT which is higher than the required voltage of 1.8V thus needing a post regulator which converts the switched capacitor output voltage of 2.15V to 1.8V. If the post regulator is a linear regulator, then the difference in voltage of the output of switched capacitor converter and the required regulated output voltage is dissipated, in the example (2.15V−1.8V)*Load current, as heat, thus decreasing the efficiency of the converter.
In FIG. 2A switch S1 202, and switch S4 206 connect to one side of capacitor C1 204. The other side of capacitor C1 204 connects to switch S2 203 and switch S3 205. The other side of switch S1 202 connects to VIN 201. The other side of switch S 203 connects to ground. The other side of switch S3 205 and switch S4 206 each connect to VOUT 208 which is connected to capacitor C2 207 and LOAD 209. The other side of C2 207 and LOAD 209 each connect to ground.
For example, in the switched capacitor converter of FIG. 2B, the output voltage VOUT 217 is equal to the twice the input voltage VIN 210.
In FIG. 2B switch S1B 212 and switch S2B 211 are each connected to VIN 210. The other side of switch S1B 212 is connected to one side of capacitor C1B 214 and switch S4B 216, and the other side of capacitor C1B is connected to the other side of switch S2B 211, and switch S3B 213. The other side of switch 213 is connected to ground. The other side of switch S4B 216 is connected to capacitor C2B and VOUT 217. The other side of capacitor C2B 215 is connected to ground. Load 218 on one side is connected to VOUT 217 and on the other side to ground.
FIG. 2C illustrates an example clock waveform Phi1 219 and Phi2 220 for a switched capacitor converter.
FIG. 3A and FIG. 3B illustrate cascaded converters consisting of two switch mode converters cascaded in series such that the output of the first switch mode converter generates an intermediate voltage which is in between the input voltage and the required output voltage. This intermediate voltage acts as the input to the second stage. FIG. 3A shows a step down converter which could be used when the input to output ratio is wide. 301A is a switched capacitor converter which converts voltage at VIN 303A to a lower value equal to, as an example, VIN/3 at the intermediate node VINT 315A. The inductor based buck converter 302A steps down the intermediate voltage at VINT 315A, in this example equal to VIN/3, to the required voltage. Since the inductor based second stage 302A is operating from a lower voltage at VINT 315A compared to VIN 303A and since switching losses are proportional to the square of the switching voltage, the second stage converter could be run at a higher frequency than a switcher running directly from the input VIN 303A. Because of this higher switching frequency, the inductor L1 320A could be made smaller compared to a single stage inductor based switcher. Despite the benefit of the smaller inductor, the two stage conversion still suffers from the efficiency loss which is the product of the individual efficiencies of each stage. As an example if the switched capacitor stage has an efficiency of 90% and the inductor based second stage has an efficiency of 90%, then the overall efficiency is the product of the two, thus equal to 0.9*0.9=81% which is lower than the efficiency of a single stage conversion. Furthermore, the two stage conversion requires additional components like CINT 314A to hold the intermediate voltage VINT 315A and power devices 318A, 319A compared to a single stage conversion. Further, if the voltage at VIN 303A is closer to the required voltage at VOUT 322A, for example generating 3.3V output from a 3.7V Li-Ion battery, then the two stage conversion doesn't offer any benefit of decreased magnetic inductor size as the voltage on the intermediate node VINT 315A will now be similar in value to VIN 303A.
As illustrated in FIG. 3A switched capacitor array 301A has switches S1A 304A, S2A 305A, S3A 307A, S4A 308A which are connected to capacitor C1 306A as illustrated. Switches S3A 307A and S4A 308A are connected to switch S1B 309A. Switches S1B 309A, S2B 310A, S3B 312A, and S4B 313A are connected to capacitor C2 311A as illustrated.
Switches S3B 312A and S4B 313A are interconnected to VINT 315A as is CINT 314A as illustrated which connected to Inductor based Buck Converter 302A.
As illustrated in FIG. 3A Inductor based Buck Converter 302A has transistors 318A and 319A having gates 316A and 317A respectively. Inductor L1 320A is connected to capacitor C3 321A and VOUT as illustrated.
VOUT 322A is connected to LOAD 323A.
FIG. 3B illustrates a cascaded two stage step up converter taking as an input VIN 303B. At 301B is an Inductor Based Boost Converter and at 302B is a Switch Capacitor Array.
301B an Inductor Based Boost Converter has an inductor L1 305B coupled to transistors 306B and 307B as illustrated. Transistors 306B and 307B have gates 304B and 308B respectively. Transistor 307B is coupled to VINT 310 and CINT 309B.
302B a Switch Capacitor Array is coupled to VINT 310B and has switches S1 312B and S2 311B. Switch S1 312B is coupled to C1 314B and switch S4 315B. Switch S2 311B is coupled to capacitor C1 314B and switch S3 313B as illustrated. Switch S4 315 is coupled to capacitor C1 314B and VOUT 317B as illustrated.
VOUT 317B is coupled to capacitor C2 316B and LOAD 318B.
Thus there is a need for a power supply which is compact, efficient, which offers wider bandwidth without impacting conversion efficiency and which is integration friendly with the components it is powering. This presents a technical problem for which a technical solution using a technical means is needed.